Packing materials are widely used to prevent fluid leakage around an operating member in a housing with fluid, such as a rotary shaft or a sliding stem in fluid control valves or in a reciprocating pump shaft. Normally such packing is formed of a resilient member and is placed under a static load by being bolted into position within a packing box around the operating member. In other instances the packing is subjected to spring loading in what is known as a live loaded packing configuration. Live loaded packing is particularly useful in attempting to prevent leakage of undesired fluids into the environment. Also, at operating temperature conditions of around 450.degree. F. (232.degree. C.) (i.e., operating temperature in the packing area) it is desired to use commonly available graphite packing rather than packing material formed of polytetrafluorethylene (PTFE) because PTFE packing tends to extrude at such elevated temperatures which could lead to packing failure and fluid leakage.
As an example, certain applications of a fluid control valve require not only that the valve meet stringent leakage requirements but that it also meet a stringent fire retarding test to prevent catastrophic packing failure. Fluid control valves in pipelines at refineries, and in other chemical processing applications are desired to have substantially zero leakage (i.e., less than 500 ppm) of fluid around the top of the valve, and to meet fire retarding tests as an equipment safety factor.
In such applications, graphite packing alone is not suitable. Attempts to increase the loading on the graphite packing provides a slight reduction in leakage but leads to extrusion of the graphite along the sliding stem or rotary shaft operating member, thereby causing increased friction and undue limitations in the useful valve life or valve stem/shaft travel. To reduce the leakage, it is desired to use PTFE packing material which can provide a better seal than graphite material. However, the use of PTFE packing at elevated packing temperatures is normally not recommended, and particularly where the valve must meet fire retarding tests, as in refinery applications, the extrusion and potential breakdown and vaporization of PTFE packing under high temperature fire conditions would lead to catastrophic packing failure and undesired hazardous fluid leakage.